The invention relates to an electrical machine having a rotor which contains a superconducting rotor winding which is cooled to cryogenic temperature during operation.
Electrical machines, to be precise generators or electric motors, for example synchronous machines, can nowadays be equipped with superconducting components, for example a rotor winding. A cooling system is required in order to cool the rotor winding down to the operating temperature. A normal embodiment of such cooling systems uses, for example, a so-called thermosiphon and operates with a two-phase cooling medium, for example helium, hydrogen, neon, nitrogen or argon. In general, a corresponding rotor has a cooling chamber which is arranged in the area of the rotor winding and into which liquid cooling medium is introduced, where it is vaporized for cooling and is then fed back to a cold head in order to once again liquefy the cooling medium.
When the machine is being started up, all the components of the machine and of the cooling system are initially at room temperature. After the cooling system has been switched on, the so-called cold heads are cooled down first of all, and the liquefaction of the working gas starts when the temperature is sufficiently low.
Since the density of the working gas increases as the temperature falls, and in particular when it is liquefied, it is known for an appropriately large gas volume to be kept available at room temperature in an external supply container which is fitted to the cooling system in order to ensure that a sufficient amount of liquefied working gas, that is to say liquid coolant, is available during operation of the machine. In the equalization pressure container, the cooling medium is generally at ambient temperature in the gaseous state both during operation and when the machine is at rest. A buffer system such as this results at least in a system which operates “at the push of a button”, that is to say the user does not need to worry about replenishment of cooling medium, or allowing cooling medium to escape. In the end, he just has to switch on the machine, including the cooling system, after initial filling.
In order to keep the dead volume, that is to say the cooling circuit volume which is actually not required during operation in an arrangement such as this, small, it is desirable for the initial pressure at room temperature to be as high as possible. The equalization vessel is therefore generally in the form of a pressure container. The equalization pressure container is arranged outside the electrical machine and is connected via an appropriate connecting line to the cooling system, which leads into the rotating rotor of the machine.
Alternatively, it is known for the working gas to be replenished from a reservoir when the machine is being started up. This means that the installation operator has to ensure that the gas is replenished.
When the machine is shut down, the vaporizing working gas must then once again be stored in the supply container, for example a gas cylinder or a pressure vessel, or alternatively must be blown out. Particularly during replenishment and when working gas is escaping, the user must ensure the supply of working gas. In addition, the working gas is generally expensive, which means that blowing out should be avoided. It is therefore also known for the blown-out working gas to be fed back again into a supply cylinder, but this requires the use of an external compressor.